Comets and the Age of the Solar System

Abstract

The existence of comets as an argument for a recent creation is
examined. Most creationist presentations of this topic are out of date. To rectify
this situation, the tremendous amount of work on the origin and evolution of
comets by evolutionary astronomers over the past two decades is reviewed. While
it was once thought that the Oort cloud could account for all comets, computer
simulations have clearly shown that short-period comets cannot originate from
the cloud, so the Kuiper belt has been revived to explain the origin of the
short period comets. The alleged discovery of the Kuiper belt is discussed,
while the status of the Oort cloud as a theory is questioned. It is concluded
that the existence of comets is still a valid argument for a recent creation
of the Solar System.

Introduction

The existence of comets has long been used as an argument for
a recent creation (probably the best treatment so far is that of Slusher1).
The case is usually made as follows. The standard model of a comet is one in
which all of the material observed is released by an icy nucleus only a few
kilometres across. This model strongly suggests that comets are very fragile,
losing much of their material during each close pass to the Sun. Most comets
follow orbits that take them vast distances from the Sun. If a comet’s
orbit takes it too far from the Sun, then the comet could easily be captured
by the gravitational attraction of other stars and thus would be lost to the
Solar System. This places a maximum distance from the Sun that a comet may orbit.
If this maximum distance can be estimated, Kepler's third law of planetary motion
can be used to deduce the greatest possible orbital period that a comet may
possess (about 11 million years). When combined with an estimate of how many
trips around the Sun that a comet can survive, we can estimate the maximum age
of comets. This figure is far less than the adopted 4.6 Ga age of the Solar
System. Because no source of creation for comets has been identified, comets
are assumed to be primordial. If this is true, then the age of the Solar System
must be less than the estimated upper age of comets.

This has been recognised as a problem in astronomical circles
for a long time. There have been several suggested resolutions to this problem,
the most popular and successful being that of the late Dutch astronomer Jan
Oort.2 Oort proposed a large spherical
cloud of comet nuclei that formed early in the history of the Solar System.
The Oort cloud is supposed to be at a large distance from the Sun, placing the
nuclei too far away to be observed. The estimated radius of the cloud has varied
over the years, and even from author to author. The inner cloud, where most
of the nuclei reside, is believed to have a radius of 10,000 to 20,000 AU.
An AU (Astronomical Unit) is the mean distance between the Earth and Sun, and
is roughly 1.50 x 108 km. Estimates of the size of the outer
Oort cloud vary, with a range of 40,000 to 150,000 AU from the Sun. At
such great distances the temperature is so low that the nuclei can be preserved
in a 'deep freeze' sort of environment so that they survive to today. Occasional
gravitational effects of other stars, called perturbations, are believed to
cause some of these nuclei to plunge toward the Sun and continue to orbit until
they are exhausted in a time-scale much less than 4.6 Ga as mentioned above.
Therefore this model suggests that all of the comets observed today have been
in their current orbits for only a fraction of the age of the Solar System.

The basic calculations and arguments within a recent creation
framework were done nearly 25 years ago. In the ensuing years no new work or
updating has been done, although the argument has been repeated many times.
During this same time astronomers have refined the Oort cloud hypothesis, though
recent creationists have not noted this. Refinements include the consideration
of periodic impacts causing mass extinctions in the past, which has become referred
to as the Alvarez3 hypothesis.
Furthermore, a related idea called the Kuiper belt has been identified as the
source of many comets, though almost no creationist writers have even acknowledged
it. Therefore, the recent alleged discovery of the Kuiper belt4
caught many creationists off guard. What is presented here is a new evaluation
of this topic, which will readdress the question of what comets tell us about
the age of the Solar System. We will also examine the alleged confirmation of
the Kuiper belt.

What are Comets?

Figure 1: Structure of a comet. The nucleus is a few kilometres
across, while the coma is about 10,000–100,000 km wide.

The word ‘comet’ comes from the Greek komhth comètè
(long-haired), from which we also get the word ‘comb’. Loosely,
a comet appears as a hairy star. Four millennia comets have been associated
with
disasters. Two examples are the apparitions of Halley’s Comet during
the Battle of Hastings in 1066 and the destruction of Jerusalem in AD 70.
Comets really do appear mysterious. While the stars, the Sun, the Moon, and
the five
naked eye planets all follow regular and predictable motions, comets appear
suddenly, quickly move in an erratic fashion, and then abruptly disappear,
apparently
never to be seen again.

It was not until the adoption of Newtonian mechanics three centuries
ago that Edmund Halley was able to show that comets do follow predictable orbits
around the Sun. Halley computed orbits for about two dozen comets that he or
others had observed. Of particular interest is the comet that Halley observed
in 1682. When he noticed that the comet’s orbit closely matched the orbits
of similar comets seen in 1531 and 1607, he realised that this comet must have
a period of nearly 76 years, that is, three comets were actually a single comet
observed during three consecutive apparitions. Since Halley’s time his
comet has returned four times, most recently in 1986. Of course, this is the
famous comet that bears his name.

Several models of what comets are have been proposed, but the
standard model for several decades has been the icy conglomerate model, or the
‘dirty iceberg’ theory of Fred Whipple.5
The term ‘dirty iceberg’ refers to the nucleus, from which material
is removed and caused to glow, making the comet visible. The nucleus of a comet
is believed to be a mass of ice several kilometres across with an admixture
of small dust particles (Figure 1). The ice consists of
various frozen materials, mainly water, carbon dioxide, methane, and ammonia.
At about 40 km in diameter, the nucleus of Comet Hale-Bopp seen in 1997
is one of the largest nuclei ever observed, which explains why it has been termed
‘a great comet’. For comparison, the bright Comet Halley has a nucleus
about one fourth that size. When far from the Sun the nucleus of a comet is
at a sufficiently low temperature for the ice to remain frozen, and thus the
nucleus can exist indefinitely in this state. As the nucleus passes close to
the Sun, the greatly increased radiation heats it so that the ice begins to
sublime. The Giotto spacecraft passed close to the nucleus of Halley’s
Comet in 1986 and revealed a surface as dark as coal. Presumably this is a crust
consisting of carbonaceous dust left behind as the ice sublimes. A similar thing
can be observed in winter in cooler climates where snow is ploughed into large
piles in parking lots. As the snow melts or sublimes, dirt is left behind to
form a dark crust on the surface. This dark coating allows for more efficient
absorption of the Sun's rays so that the sublimation of ices occurs more rapidly.
Before and during its first pass near the Sun a comet’s nucleus is expected
to be lighter in colour, but the formation of the dark crust should make the
nucleus darker on subsequent passes.

As the gas is released it rapidly expands into an envelope up
to tens of thousands of kilometres in diameter called the coma (see Figure
1 again). The Sun’s radiation ionises the gas, and the recombination
of the atoms, along with the reflection of sunlight off dislodged dust particles,
makes the coma visible. The coma is the brightest part of a comet and gives
a comet its hairy appearance, but is very tenuous as shown by the fact that
stars viewed through it are not appreciably dimmed. Subsurface sublimation results
in explosive release of gas in the form of jets, which can cause large changes
in the brightness of the coma. The solar wind shoves the ionised gas away from
the Sun, forming an almost straight ion, or gas, tail (Figure
1). The Sun's radiation pushes the more massive dust particles outward,
producing a more gracefully curved dust tail. Both tails point away from the
Sun, whether the comet is approaching or leaving the Sun.

As mentioned previously, the brightness of a comet is determined
by how bright the coma is. The brightness of the coma critically depends upon
the size and composition of the nucleus and how close it is to the Sun. An additional
factor affecting how bright a comet appears to us is how far from the Earth
it is. Generally, a comet is brightest near perihelion, the point of closest
approach to the Sun. That is why Halley’s Comet was a disappointment to
the general public in 1986. When Comet Halley was near perihelion it was on
the other side of the Sun, and hence not visible. Because of its motion relative
to the Earth the comet emerged gradually from behind the Sun. Even then it was
far away from the Earth, and given its position well below the Earth’s
orbit, it was best seen from the southern hemisphere. By the time it was visible
in the northern hemisphere it had dimmed even more. In nearly 2,300 years of
observations of this comet, the circumstances of the most recent apparition
were the absolute worst possible.

Given the size of Comet Halley’s nucleus and the observed
mass loss during its recent apparition, it is obvious that this comet cannot
sustain many trips around the Sun. From historical data it is difficult to determine
if Comet Halley has dimmed over the past 2,300 years, but it is expected to
have been slightly brighter during past visits. Nor is this behaviour unique.
Other shorter period comets have been observed to dim remarkably over the years.
Some that once produced noticeable comas show very little activity now. In fact,
the colours and orbits of some asteroids suggest that they may be burned out
remains of dead comets. Halley’s Comet has exhibited one of the slowest
decreases in brightness, probably because it has an unusually large nucleus
and is probably pretty young, even by recent creation standards. On the other
hand, a comet may also be disappointing during its first pass or two around
the Sun. Recall that the nucleus of a comet is believed to be light in colour
at first, but acquires a darker colour as dust accumulates on the surface. Because
darker coloured objects are better absorbers of radiation than lighter ones,
a darker nucleus should be heated more, which results in more sublimation and
coma formation. This suggests that new comets may not reach full potential brightness
during their first pass, but may achieve maximum brightness on their second
or third pass around the Sun. Comet Hyakutake, which was visible in 1996, had
a small nucleus, but was bright for its size. This has caused some to suggest
that this is a young comet on its second or third pass near the Sun. Comet Kahoutek
that was visible in 1973 and 1974 failed to brighten as originally anticipated,
suggesting that it may have been a comet on its first trip to perihelion.

Individual comets obviously have very limited life-times, but
is this true of comets collectively? Comet Halley, as well as other comets,
may have only been orbiting in its present orbit for only a few thousand years.
While the planets follow nearly circular orbits, comets follow very elliptical
orbits. This causes them to cross the orbits of most of the planets, and result
in a very real possibility of passing close to one or more planets eventually.
Such a pass may cause a gravitational interaction (called a perturbation) that
changes the orbit of a comet. This is particularly true of Jupiter, which has
more mass than all of the other planets combined. Perturbations can cause huge
changes in a comet’s orbit. A good example was Comet Shoemaker-Levi, which
collided with Jupiter in the summer of 1994. The collision was caused by a near
miss of Jupiter which the comet had experienced about two years earlier that
had placed the comet in a radically different and doomed orbit.

It is believed that periodic comets like Halley’s Comet
once followed a much larger, more elliptical orbit. Chance encounters with Jupiter,
and to a lesser extent the other planets, have changed its orbit to the present
one. If this is true, Comet Halley may have been in its current orbit for as
little as 3,000 years. In addition to a smaller orbit, an interaction of a comet
has a nearly equal chance of resulting in its complete ejection from the Solar
System, and a small probability of its complete destruction via an impact, such
as Comet Shoemaker-Levi endured. It appears that all short period comets experience
chaotic orbits, that is, they have orbits that are extremely unstable and undergo
relatively rapid and large changes.

Comet Orbits

It is obvious that periodic comets must be replenished, or else
they would be exhausted in thousands of years. Each year a number of new comets
are discovered (recently this has been about two dozen per year). Most of these
comets are relatively faint, but occasionally a brighter one is found. Two recent
bright comets were Hyakutake in 1996 and Hale-Bopp in 1997. These two comets
were the brightest ones seen in 20 years, with Hale-Bopp considered to be a
‘great comet’.

Like all orbiting bodies, comets have elliptical orbits. Recall
that an ellipse is a conical section possessing two foci and having the property
that the sum of the distances from the two foci to any point on the ellipse
is constant. The longest diameter of an ellipse is called the major axis, while
the smallest diameter is called the minor axis. The size of an ellipse is defined
by the length of the major axis. Ellipses vary in shape from the circle to very
flattened ellipses, where the minor axis is much smaller than the major axis.
The measurement of the flattening of an ellipse is called the eccentricity,
and is defined as the ratio of the distance between the foci to the major axis.
A circle has an eccentricity of zero, but the eccentricity of an ellipse may
have any value of up to, but less than, one. The conical sections that have
eccentricities of one and greater than one are the parabola and the hyperbola,
respectively. Objects may follow parabolic or hyperbolic orbits, but since these
two figures are not closed, any objects having such orbits will pass the Sun
once and never return, and hence would not be permanent members of the Solar
System.

Planetary orbits tend to be nearly circular, while comet orbits
tend to be very elliptical. Newly discovered comets are frequently observed
to have eccentricities of one, which on its face would suggest that they are
mere visitors to the Solar System. However, eccentricities can be measured only
to about four significant figures, so that it is quite likely that the eccentricities
are actually slightly less than one, and the differences from one are masked
by the observational errors. This means that all comets are members of the Solar
System, but the very largest, most elongated orbits are observationally indistinguishable
from parabolas. The comets following these orbits would have periods of many
thousands, if not millions, of years.

The orbits of more than 600 comets have been computed, and several
important clues about the nature and origin of comets can be gleaned from them.
First, no comet has been observed to have a hyperbolic orbit approaching the
Sun, though some have been observed leaving the Sun in such an orbit. This strongly
suggests that all comets are permanent members of the Solar System. If comets
had an interstellar origin, we would expect that many would be approaching with
hyperbolic orbits. The ones that leave with hyperbolic orbits do so after having
an interaction with one or more of the planets, and this represents one of the
cometary loss mechanisms.

A second clue is that there is a general division between comets:
short-period, with periods less than 200 years, and long-period, with periods
longer than 200 years. There are about 100 short-period comets, and more than
500 known long-period comets. This division is not an arbitrary one in time,
for the typical orbits of the two groups are quite different. Most short period
comets orbit in the prograde direction, that is, the same direction that all
the planets revolve around the Sun. About half the long-period comets orbit
prograde, while about half have retrograde orbits. Most short-period comets
also have low inclinations, which means that the planes of their orbits are
tilted very little with respect to the orbits of the planets. Long-period comets
may have almost any inclination. One notable exception is Halley’s Comet;
while it has a period of less than 200 years, its orbit is highly inclined and
is retrograde. This could suggest that Comet Halley was originally a long-period
comet that recently experienced a strong perturbation which converted its orbit
into one having a much shorter period. It must be emphasised that while the
orbits of the two groups of comets are quite different, the physical properties
of the two groups, such as composition, are identical. This suggests that all
comets have a common source.

What is the maximum period that a comet may have? The gravitational
forces of nearby stars impose an upper limit to the size that an orbit may have.
If the aphelion (the point of maximum distance from the Sun) is a significant
fraction of the distance to the nearest stars, the comet has a large probability
of being removed from the Sun's grip. Let us adopt a liberal aphelion distance
of 100,000 AU, which is more than one-third the distance to the nearest
star. The semi-major axis would be 50,000 AU. The semi-major axis and orbital
period are related by Kepler's third law of planetary motion:-

a3 = p2

where a is the semi-major axis in AU, and

p is the period in years.

A 50,000 AU semi-major axis results in a period of 1.12 x
107 years. If a comet has followed this orbit for 4.6 Ga, it
would have experienced more than 400 trips around the Sun. After that many perihelion
passages it is doubtful that there would be any volatile material left in the
nucleus. Note that 50,000 AU figure was a very liberal upper limit, and
so most comets would have orbited far more times. A more realistic estimate
of the upper limit at 25,000 AU for a semi-major axis for a stable orbit
yields a period of 3.95 x 106 years, with a result of almost 1,200
returns in 4.6 Ga.

While most creationists’ writings have focussed on evaporation
of volatile materials from the nuclei as the loss mechanism for comets, at least
two other loss mechanisms are known. One of these is the ejection from the Solar
System by close planetary interactions, and the other is collisions with planets.
While direct collisions are considered to be relatively rare fates for comets,
some recent studies have suggested that ejection may play a more important role
than disintegration. It appears that if comets are primordial there should not
be any left.

What is the Oort Cloud?

So what source of comets do evolutionists propose? Several sources
have been suggested over the years, and have largely fallen into disfavour.
For instance, nearly two centuries ago, Laplace suggested that comets might
be interstellar, with comets occasionally passing near the inner Solar System
so that they become visible and some would be captured. One would expect that
at least a few comets would be observed approaching perihelion on hyperbolic
paths. As mentioned previously, this is not the case, which is the main reason
this model was largely abandoned. Apparently this difficulty can be explained,
at least to the satisfaction of its few adherents today (see, for example, Witkowski6).
Another suggested source of comets is by volcanic ejection from planets and
their satellites (Vsekhsvyatskij7).
An obvious problem with this idea is that comets appear to share a common composition,
a property that is not true of the alleged parent bodies. Another problem is
the difficulty of ejected objects to assume the orbits observed. Today van Flandern8
is the champion of the hypothesis that comets originated from the disruption
of a planet between the orbits of Mars and Jupiter. This hypothesis has its
own problems and has not been accepted by many people.

The vast majority of astronomers today believe the hypothesis
of Oort, who suggested a vast reservoir of comet nuclei at a great distance
from the Sun to be the source of new comets (Figure 2).
This proposal was not exactly a shot in the dark as many seem to believe. Instead,
it was based upon careful study of the semi-major axes of the orbits of long
period comets known at that time. A histogram of 1/a0 shows a cluster
near 1/a0 = zero. Oort reasoned that this clumping at great distance
represented the original distribution of comets, while the smaller numbers at
closer distances represented the result of gravitational perturbations.

Some could criticise Oort’s histogram on the basis that
a plot of 1/ao amounts to a logarithmic plot of distance, and hence
would include an ever increasing volume of space as 1/a0 approaches
zero. But this is irrelevant, because what is being plotted is the frequency
of total energy. Since the only conservative force involved is gravity, and
gravity goes as the negative of the inverse of the distance, this is the proper
plot. On the other hand, one could criticise this approach by pointing out that
the lower energy orbits are far more likely to suffer loss through the mechanisms
previously discussed. There are two reasons for this. First, the comets following
the smaller orbits would visit the inner Solar System more often, leading to
more frequent volatile material loss near perihelion. Second, the increased
number of trips through the region where the planets are found, and at lower
speeds than the longer period comets, would lead to more frequent perturbations
caused by the major planets, leading to an increased chance of ejection. Thus
it would seem that the lower energy comets should be selected for loss over
the higher energy ones. If this is correct, then one would expect that any distribution
in energies would eventually result in the observed histogram.

With the assumption of evolution, the Solar System is believed
to have formed from the collapse of a large cloud of gas about 4.6 Ga ago.
Most of the material is supposed to have fallen to the centre to form the Sun,
while the remainder flattened to a disk, from which the planets eventually formed.
The first step in forming the planets was the material coalescing into small
chunks called planetesimals. These gradually accreted until a few were large
enough to stay together by gravitation and to begin attracting other planetesimals
by gravity. The larger of these eventually became the planets, with leftover
material becoming the satellites and asteroids. The regions nearer the protosun
would have been warmer, and hence the lighter elements would have been evaporated
and removed from the inner Solar System, while the outer regions, being cooler,
would have retained volatile material. This is supposed to explain why the inner
planets have a rocky composition (lacking lighter elements) and the outer planets
have a lighter element composition. This also demands that the comets originated
far from the Sun, because they are made mainly of lighter elements.

The gravitational perturbations of the planets would supposedly
have removed most of the leftover planetesimals in the region of the Solar System
occupied by the planets. The primary mechanisms for removal would have been
ejection and collision. Indeed, the many craters observed on the surfaces of
most of the smaller bodies of the Solar System are believed to be the results
of these collisions. The asteroid belt is largely populated by small bodies
(planetesimals) that are located in stable orbits controlled by the planet Jupiter.
Jupiter has such a strong influence because it has more mass than all the other
planets combined. At the distance of the asteroid belt from the Sun, the temperature
would have been sufficient to remove much of the lighter elements. Indeed, any
left-over planetesimals that orbit closer to the Sun than the asteroid belt
tend to have rocky composition, while it is expected that more distant ones
would tend to have lighter element composition.

Good summaries of the modern view of the Oort cloud are given
by Everhart9 and Weismann,10
and are briefly described here. The aphelia of the cometary nuclei in the Oort
cloud would extend no more than 50,000 AU (one-fifth the distance to the
nearest stars), or else the nuclei would likely be lost to the Sun. The perihelia
would come no closer than 30 AU from the Sun. This would place the perihelia
beyond the orbit of Neptune, and hence out of the planetary region and immune
to large perturbations. Nuclei in such orbits should experience little dissipation,
and so the cometary system should exist over several Ga. While Oort originally
envisioned stellar perturbations to be the major factor in changing the nuclei’s
orbits, it is now realised that interstellar gas clouds11
and galactic tides12 are major
contributors as well. In fact, it now appears that the classic Oort cloud beyond
20,000 AU is not stable over 4.6 Ga. That is, the perturbing forces
should have dissipated that cloud by now. Therefore it is now hypothesised that
there is an inner and outer Oort cloud. While the outer, classic, Oort cloud
is depleted, it is replenished by gradual elevation of inner cloud objects into
the outer cloud.

The total energy of an orbiting body is the sum of kinetic and
gravitational potential energies. All bound orbits have negative total energy,
but a typical orbit described above would have total energy very close to zero.
According to Everhart, a stellar perturbation near aphelion usually results
in an energy loss. Since the aphelion distance is unaffected and potential energy
depends upon distance, the potential energy remains constant. Thus the decrease
in total energy is manifested as a decrease in kinetic energy, causing the aphelion
speed to decrease. This decrease in speed results in a smaller perihelion distance,
which can bring a comet nucleus into the planetary region. If the perihelion
distance is greater than 5 AU, there is little solar dissipation. Such
objects are rarely discovered, because they fail to produce noticeable comas.

While solar dissipation for such objects is negligible, for perihelion
distances between 5 and 30 AU planetary perturbations are quite significant.
About half the perturbations will result in a gain in energy, causing these
comets to be removed from the Solar System. The other half will lose energy.
The energy loss occurs near perihelion, and can be approximated by a single
loss each orbit. The perturbations do not appreciably change the instantaneous
distance from the Sun, so the gravitational potential energy is unchanged. Therefore
any loss in energy again must be from kinetic energy. Since the perihelion distance
remains constant, a decrease in total energy lowers the aphelion distance, and
hence the orbital period as well. Comets that take this route enter a regime
of unstable orbits, with many perturbations. Comet Hale-Bopp would be classed
in this type, for it recently entered the planetary region in its recent pass
with a period of about 4,200 years, but it left with a period of about 2,600
years. Further interactions in these unstable orbits involve many possibilities,
including a return to the Oort cloud.

A little known detail is that the Oort comet cloud was devised
to explain the long period comets, though many have assumed that it explained
the short period comets as well. Oort himself apparently believed that the short
period comets were best explained by the disruption of a planet that once orbited
between Mars and Jupiter, an old idea that has been largely discarded, but still
has its supporters (for instance, van Flandern). The problem is the significant
differences in orbits between the two types of comets. Many have assumed that
gravitational perturbations can transform long period comets into short period
ones, but recent calculations have revealed that this is unlikely.13
Tremaine et al.14 showed
that perturbations on a collection of nuclei with a random distribution of inclinations
would preserve the inclinations, that is, the random distribution in inclination
would remain random. In fact, prograde, low inclination orbits are more susceptible
to perturbations, because these orbits allow for greater time of interaction
between the comets and planets. Since short period comets have low inclination,
prograde orbits, there must be a source for short period comets other than the
Oort cloud.

The Kuiper Belt

Figure
2: The sun and planets lie on a flat disk; the hypothetical Kuiper
belt is doughnut shaped and centred on the sun, while the hypothetical
Oort cloud is a thick shell and also centred on the sun. Return
to ‘What is the Oort Cloud?’

Beyond the orbit of Neptune, the perturbations of Jupiter would
have had little effect during the formation of the planets, while the planetesimals
near the orbits of Jupiter and Saturn would have been ejected. This realisation
is what caused Kuiper15 to suggest
in 1951 that the solar system does not end abruptly beyond Neptune and Pluto.
Since no planets are found beyond these orbits, any material there must be in
the form of planetesimals. Some have renamed these distant planetesimals ‘cometesimals’,
because they would be the comet nuclei.16
For many tens of AU beyond the planets, any planetesimals would have reasonably
stable prograde orbits with low inclinations, with compositions similar to the
major planets. Gradual accumulation of small perturbations on these bodies would
cause either increases in aphelia or an infall into the inner Solar System.
The latter result would produce objects with the properties of the short period
comets. Because of its flattened distribution this reservoir has been called
the Kuiper belt (see Figure 2 again).

For years the Kuiper belt was mostly overlooked in deference to
the Oort cloud. It was believed that the Oort cloud could account for both long
and short period comets, and so the Kuiper belt was viewed as unnecessary except
possibly as the inner portion of the Oort cloud. However, computer simulations
done during the 1980s revealed that the Oort cloud could not produce enough
short period comets with the required low orbital inclinations. The problem
is that the process of converting long period comets into short period comets
is not efficient enough to deliver any significant number of comets before their
disintegration or expulsion from the Solar System. In the past 15 years the
Kuiper belt has been resurrected as the source of short period comets, though
this escaped the notice of most creationists.

The resurgence of the Kuiper belt spurred a concentrated search
for objects orbiting in the belt. The recent announcement of the alleged discovery
of the Kuiper belt17 has sparked
much interest, though some are rightly concerned if the observations are real.18
Any comet nuclei in the belt would be very faint, but perhaps the brightest
ones could be photographed with the CCD (Charge Coupled Device) camera of the
HST (Hubble Space Telescope). Because any nuclei would be very faint, very long
exposures were required. The exposure times were long enough so that the orbital
motions of any photographed nuclei would trail their images. This trailing smears
the images, making them appear even fainter. The same problem has long been
encountered in searching for minor planets (or asteroids) using ground-based
telescopes. The solution is to calculate the orbital motion for an object in
the location in which you are searching and to move the telescope at the same
rate to compensate. Any orbiting target objects appear as points, while stars
are trailed. In the case of the HST observations, 34 CCD images were made of
a small part of the sky. The region photographed was selected for two properties:
lying along the ecliptic to avoid most other Solar System objects, and for containing
very few stars or galaxies to ease analysis. Each image was about ten minutes
long with a total exposure of about five hours over a 30 hour time period.

The 34 exposures made with the HST revealed more than 50 faint
point-like objects at the limit of the detectability of the system, and these
objects were deemed to be Kuiper belt candidates. A major problem is that the
imaging system is subject to random signals, called noise, that mimic these
faint points, so from a single image one cannot have any confidence that any
particular point of light is real. To sample the noise level a number of exposures
were made as the HST was moved in the opposite direction so that star images
were smeared as before, but comet nuclei images would be smeared twice as much
as the stars and thus would not be detectable. Any point sources now observed
must be noise, and so their count was taken as the noise level. This number
was a little more than half of the number of candidates, and it was concluded
that the difference, about two dozen, was the number of nuclei discovered.

As mentioned before, another study using ground-based instruments
failed to confirm the findings of the HST observations, and so a team will attempt
to repeat the observations soon.19
Another disturbing aspect of such statistically based arguments is that no one
can clearly identify a single image as a comet nucleus. It would be almost as
if an astronomer pointed out a half dozen star-like objects in the sky and announced
that he is very confident, say 95 %, that at least one of them is a planet,
but he cannot tell for certain which one is indeed a planet. Most would find
such a prospect absurd, but this is increasingly the sort of thing encountered
as high-powered statistical methods have been applied in astronomy. This is
reminiscent of the 1992 announcement of the fluctuations allegedly discovered
in the cosmic background radiation.20
The researchers in that study admitted that they could not point to any location
on their map and say ‘this is one of the fluctuations’, but
they were convinced that the fluctuations were real. Is this what science has
become?

One could respond that in the health sciences such statistics
are used all the time. For instance, over the past 30 years statistical studies
have established a clear link between cigarette smoking and certain lung diseases,
such as cancer and emphysema. The tobacco industry has responded that in any
individual case of lung disease it cannot be proven that smoking definitely
caused the disease. This is true, because non-smokers occasionally develop these
diseases as well, and so it is possible that the smoker may have developed the
disease regardless of tobacco use.

But such an analogy to the discovery of comet nuclei would be
improperly applied. What is alleged here is detection, not correlation. The
proper analogy, if it is to be made, would be to question the diagnosis of disease.
That is, a physician would have to state that he cannot definitely identify
a single case of lung disease, but that given a large enough sample he can state
with some confidence that he is examining some number of diseased lungs. Of
course this is not what is claimed, because x-rays, CAT scans, biopsies, and
finally post-mortem examinations can identify diseases with virtual 100 per
cent confidence in every case.

Perhaps time will reveal if the alleged discovery of Kuiper belt
comet nuclei is real, but the problem has been approached from a different direction.
In 1977 a large minor planet (eventually named Chiron) was discovered orbiting
between the orbits of Saturn and Uranus. Previously no minor planet beyond the
orbit of Jupiter had been known, though such objects should have been anticipated
since several thousand minor planets had been found in the inner Solar System.
It was later determined that Chiron had similar colour and spectrum consistent
with that of comets. In 1988 a faint coma was observed around Chiron, further
suggesting that it might be a very large comet nucleus. Spurred by this information
several astronomers began searching for other minor planets or comet nuclei
at distances beyond Saturn. Since 1990 more than three dozen objects have been
discovered, some beyond the orbit of Neptune, and more are being discovered
(a good review of this is by Luu and Jewitt,21
two of the researchers involved). It must be stressed that these objects are
very real, and orbits have been calculated for most of them. This is unlike
the previous study of the alleged discovery of the Kuiper belt, where no objects
were clearly identified and hence no orbits could be calculated.

Since the HST study these real objects beyond the orbit of Neptune
have been increasingly referred to as Kuiper belt objects. This not-so-subtle
shift should not mask several potential problems. First, there is some question
of the Kuiper belt extending all the way into the orbits of the outer planets.
Are orbits here stable on the time-scale necessary, and can these objects produce
the properties of short period comets? Second, it is assumed that the large
objects discovered here indicate that there must be many other smaller objects
as well. This assumption seems reasonable, as suggested from crater and asteroid
belt statistics which reveal an exponential increase in number as size decreases.
It must be remembered that this is an assumption, and as long as it is recognised
as such, we see no reason to challenge it. A third problem is the sheer size
of the objects involved — they are over ten times larger than
the largest observed comet nuclei. This translates into over 1,000 times the
volume and mass. It boggles the mind to contemplate the extreme brightness and
size of the resulting comets from such huge nuclei. If such nuclei are common,
why have none of these comets been seen with perihelia near the Sun?

This line of reasoning has caused a re-evaluation of the status
of Pluto. Heralded as the discovery of a ninth planet and the perturber of Neptune's
orbit in 1930, Pluto's classification is now in doubt. Even in 1930, it appeared
to be too small to account for the alleged perturbations. The discovery of its
satellite Charon nearly 20 years ago and the season of mutual eclipses of the
two bodies in the 1980s have led to very good measurements of the sizes and
masses of Pluto and its moon. The resultant densities are consistent with an
icy composition containing an admixture of rocky material, the same as cometary
composition. There is an attempt to reclassify Pluto and its moon as very large
asteroids, or, given their orbit and composition, members of the Kuiper belt.

The Interaction of the Oort Cloud and the Kuiper Belt

It is now clear that short period comets do not evolve from long
period comets, and so the two groups of comets require different sources. In
their original forms, the Kuiper belt was devised to explain the existence of
short period comets, and the Oort cloud was to explain the origin of long period
comets. While the orbits of these two groups of comets are quite different,
there does not appear to be any difference in composition between the two groups.
One could simply argue that all comets form far from the Sun so that the composition
is similar, but there is some question of how cometesimals could form at great
distances from the Sun, given the low density of material that would have been
there. Recent dynamical studies suggest that all comets could have formed in
the Kuiper belt, and that there has been a migration, or an evolution, between
the Kuiper belt, where planetesimal density would have been great enough, and
the Oort cloud.

This evolution has been reviewed elsewhere,22,23
and will be summarised here. Earlier the alleged evolution of the Solar System
was outlined. In the planetary region, planetesimals were able to coalesce into
planets and satellites. Beyond the planetary region the planetesimals failed
to coalesce, perhaps due to the lower density present there. Like all of the
other planetesimals, the distribution had flattened toward the ecliptic into
a toroidal shape. Being far from the Sun, these planetesimals retained their
volatile composition. In short, these unamalgamated planetesimals have the composition
of comet nuclei, while their orbits have a distribution similar to the Kuiper
belt. Thus the Kuiper belt represents the primordial population of comets. Other
stars are now known to possess disks of material at similar distances or greater
distances (examples are Vega and ß Pictoris).

Gradual planetary perturbations could transform Kuiper belt objects
two ways. One would be decreases in energy which would lower the perihelia into
the planetary region where planetary perturbations would be accelerated. These
comets would generally have prograde orbits, aphelia in the Kuiper belt, and
hence periods of less than 200 years. In other words, these would be the short
period comets, as originally suggested by the Kuiper belt.

The second possibility is increases in energy, which would maintain
perihelia in the Kuiper belt region, but would produce aphelia at tens of thousands
of AUs from the Sun. The resulting orbits would have very large eccentricities.
Nuclei in these orbits would spend much of the time near aphelion. This would
greatly increase the effect of stellar perturbations and galactic tides on the
orbits. These perturbations would tend to be more random and so would randomise
the comet orbits. This would lead to higher inclinations, with many orbits assuming
retrograde direction. This distribution matches the alleged properties of the
Oort cloud. Thus in this model many Kuiper belt objects are evolved into the
Oort cloud, from which further perturbations would produce long period comets.
Of course if this model is correct, then at long last an explanation of why
comets still exist in an old Solar System will have been found. But one must
remember that we have heard this sort of explanation before. For instance, just
two decades ago it was generally believed that the Oort cloud could explain
all comets, but later studies revealed that it could not produce short period
comets in sufficient numbers. The evolution of Kuiper objects into the Oort
cloud is a recent result, and must be examined further to see if it works.

Conclusion

Since the early creationist writings on comets and what they indicate
about the age of the Solar System, much work has been done from an evolutionary
stand-point. Many creationists have either ignored or remained ignorant of these
developments. These developments include the Kuiper belt, the simulated evolution
of comet orbits, and the alleged discovery of the Kuiper belt. This paper has
reviewed many of the new developments and now offers some conclusions and suggestions.

First, with the discovery of additional loss mechanisms, it is
even more obvious today that comets could not have been in their current orbits
since the beginning of the Solar System, if the age of the Solar System is on
the order of Ga. The need to explain the existence of comets in an old age framework
has spawned much theoretical research into the dynamics of cometary orbits.
While the basic concept of the Oort cloud has been retained, the idea has been
refined and expanded.

Second , it must be emphasised that the Oort cloud has not been
observed, nor is it likely to be observable for some time to come. Consider
this quote from Sagan and Druyan:

‘Many scientific papers are written each year about the
Oort Cloud, its properties, its origin, its evolution. Yet there is not yet
a shred of direct observational evidence for its existence.’24

This raises a very important question as to the scientific status
of the Oort cloud. Can something that cannot be observed, even indirectly as
in the case of subatomic phenomenon, be classed as scientific? While the Oort
cloud is often referred to as a theory, given the usual definition of a theory
and the impossibility of observation, can the Oort cloud be termed a theory?
Indeed, given that it is doubtful that this idea can ever be tested, one has
to question whether the Oort cloud is even an hypothesis.

Third, while the Oort cloud may not be observable, it appears
that the Kuiper belt may be. Though the initial announcement of the discovery
of typical, small belt objects by the HST was undermined by the failure to repeat
the result, the systematic search for large inner belt objects just beyond the
Jovian planets has apparently succeeded. These objects are the only serious
threat to the use of the existence of comets as an argument for a young Solar
System. Their orbits and inferred compositions are consistent with their identification
as comet nuclei, however their large sizes presents a problem for this view.
It is regrettable that creationists have remained uninformed about these developments,
and it is hoped that this review has helped to remedy the situation and will
encourage others to continue to stay abreast of this subject.

Fourth, while if the existence of the Kuiper belt is confirmed
it would provide a mechanism for short period comets, the untestable Oort cloud
would still be necessary for long period comets. The theoretical calculations
of the hypothesised evolution of Kuiper belt nuclei into Oort cloud objects
mentioned in the previous section is somewhat speculative. Creationists should
continue to monitor these studies, examining them for the soundness of their
assumptions and techniques. If the Kuiper belt exists, and if these simulations
are properly performed, then the Oort cloud becomes more plausible.

Of course, conducting our own simulations and calculations would
be the one sure way to test the correctness of these models. Very few creationist
writings on comets have been quantitative, and few have produced original research,
relying instead upon the (often old) quotes of non-creationist astronomers.
One exception is the paper on the lifetimes of short period comets by Stillman.25
This is a good example of the kind of work that creationists should be doing
in the field.

Evolutionary astronomers have spent much time developing scenarios
to explain the existence of comets in a 4.6 Ga Solar System. Despite this
effort and apparent progress, there are still many questions and problems. At
this time it is still quite doubtful that either the Kuiper belt or Oort cloud
exist, as they must in an old Solar System. It is concluded that comets still
offer a good argument for the recent creation of the Solar System. Creationists
are strongly advised to continue to monitor developments on the origin of comets.

References

Slusher, H.S., 1980. Age of the Cosmos,
Institute for Creation Research, San Diego, California, pp.43–54. Return
to text.

Oort, J.H., 1950. The structure of the cometary
cloud surrounding the solar system and a hypothesis concerning its origin.
Bulletin of Astronomy of the Netherlands, 11:91–110. Return
to text.

Vsekhsvyatskij, S.K., 1972. The origin and
evolution of the comets and other small bodies in the solar system. In:
The Motion, Evolution of Orbits, and Origin of Comets, G.A. Chebotarev,
E.I. Kazimirchak-Polonskaya and B.G. Marsden (Des), D. Reidel Publishing,
Dordrecht, Holland, pp.413–418. Return to text.

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Answers in Genesis is an apologetics ministry, dedicated to helping Christians defend their faith and proclaim the gospel of Jesus Christ effectively. We focus on providing answers to questions about the Bible—particularly the book of Genesis—regarding key issues such as creation, evolution, science, and the age of the earth.